It is a modern trend of the vehicles to employ a power transmission device for vehicles which automatically executes the shifting operation or the clutch operation for easy driving. One of such power transmission devices for vehicles is a power transmission device used for diesel engine-mounted vehicles, employing a transmission of the type of parallel shaft gear mechanism, arranging an automatic clutch, and interposing a fluid coupling between the engine and the automatic clutch as disclosed in, for example, Japanese Patent No. 3724491 (patent document 1) assigned to the present applicant.
The engine for the power transmission device (see FIG. 4) described in the patent document 1 is equipped with an engine control device which determines the amount of feeding the fuel (fuel injection amount) depending upon the amount the accelerator pedal is depressed and the engine rotational speed. When normally traveling, the fuel injection amount is controlled by using the amount the accelerator pedal is depressed as a basic parameter (accelerator pedal follow-up control). At the time of shifting by disengaging the clutch for shifting attempt, the engine control device assumes a control mode to control the engine (engine control at the time of shifting) independently of the amount the accelerator pedal is depressed to cope with a sharp decrease in the engine load stemming from the disengagement of the clutch and resumes the accelerator pedal follow-up control at a moment when the clutch is engaged. The change-over of the control mode is effected not only in the power transmission device interposing a fluid coupling but also in the power transmission devices for vehicles in general.
FIG. 5 is a timing diagram of a power transmission device of a vehicle (automobile) equipped with an engine control device for executing the above-mentioned controls, a clutch control device for controlling the amount of engaging the clutch and a shift control device for controlling the operation of the transmission, and shows operation characteristics of these control devices at the time of shifting. The amount of engaging the clutch is controlled depending upon the duty ratio D(%) of pulses output from the clutch control device same as the one described in the patent document 1; i.e., the clutch is completely engaged when the duty ratio is 0%, and the amount of engagement becomes zero when the duty ratio is 100% and the clutch is disengaged.
While the vehicle is traveling, for example, at the second speed in FIG. 5, if a shift instruction A is produced by the shift control device (or the shift lever operated by a driver) so as to shift to the third speed, then the clutch control device produces a duty ratio of 100% (in practice, the amount of engagement becomes zero being slightly delayed behind as shown) to disengage the clutch. At a moment the clutch is disengaged, the shift control device disengages the gears of the second speed, renders the transmission to be neutral, and brings the gear into engagement with the gear of the third speed through the synchronizing action by the synchronizing mechanism. In this step, the rotational speed of input shaft of the transmission sharply drops down to a rotational speed corresponding to that of the gear of the third speed.
Upon the output of the shift instruction, the control mode of the engine control device is changed from the accelerator pedal follow-up control over to the engine control at the time of shifting, the amount of fuel injection is decreased to meet a sharp decrease in the engine load as a result of disengaging the clutch to thereby prevent the engine rotational speed from sharply rising. In controlling the engine at the time of shifting, first, the amount of fuel injection is gradually decreased down to the amount of fuel nearly at the time of idling (graduation control) and, therefore, the amount of fuel is so controlled that the engine rotational speed approaches the rotational speed of the input shaft of the transmission at the third speed. At the time of shift down such as from the third speed to the second speed, the engine rotational speed is so controlled as to maintain the present state or to be increased by a predetermined amount.
At a moment the gears of the transmission are engaged, the clutch control device starts controlling the clutch so as to be engaged. In controlling the clutch so as to be engaged, a half-engage clutch control is executed by sharply decreasing the duty ratio down to about 0% for a short period of time (called “one-shot engagement”) and, thereafter, returning the duty ratio back to the state of a so-called half-engaged clutch, and gradually decreasing the duty ratio again. The duty ratio is set to be nearly 0% for a short period of time from the standpoint of stabilizing the operation of a hydraulic piston of the clutch (wet type multiple disk clutch) so as to quickly arrive at the state of half-engaged clutch. In practice as shown, the amount of engagement quickly arrives at the start of the state where the clutch is half-engaged. In the state where the clutch is half-engaged, the amount of engagement gradually increases nearly according to a change in the duty ratio. This avoids a shift shock or an engine stall at the time of engaging the clutch.
As the amount of engaging the clutch increases in the half-engage clutch control, the engine rotational speed further decreases while the rotational speed of the input shaft of the transmission increases; i.e., the two rotational speeds gradually approach each other. At a moment (B) when the difference has reached a predetermined value, the control mode of the engine control device returns from the engine control at the time of shifting back to the accelerator pedal follow-up control, and a control is executed by using the amount the accelerator pedal is depressed by the driver as a basic parameter. At the time of returning back to the accelerator pedal follow-up control, too, a graduation control is executed for gradually increasing the amount of fuel injection.